1. Field of the Invention
Exemplary aspects of the present invention generally relate to an electrophotographic image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photoconductive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
In known image forming apparatuses using an electrophotographic method, a transfer bias, for example, a direct current (DC) transfer bias under constant current control, is applied to the transfer device using a DC power source. Generally, under the constant-current control, an output voltage in a bias application circuit is detected by a detection circuit provided to the bias application circuit, and a resistance at a transfer roller side (a resistance including an image bearing member and a recording medium, for example), is calculated based on the detected output voltage. Based on the obtained resistance, a transfer current value is determined and adjusted (corrected). Alternatively, ambient temperature and humidity are detected, and the transfer current value is determined and adjusted based on the detected temperature and the humidity.
In recent years, a variety of recording media sheets such as paper having a luxurious, leather-like texture and Japanese paper known as “Washi” have come on the market. Such recording media sheets have a coarse surface through embossing process to produce that luxurious impression. However, toner does not transfer well to such embossed surfaces, in particular, the recessed portions of the surface. This inadequate transfer of the toner appears as dropouts or white spots in the resulting output image.
Various attempts have been made to prevent improper transfer of the toner under such circumstances. For example, a superimposed bias, in which an alternating current (AC) voltage is superimposed on a direct current (DC) voltage, is supplied as a secondary transfer bias to enhance transferability.
In such a configuration, the AC component affects transfer of toner to the recessed portions of the recording medium, and the DC component affects transfer of toner to the projecting portions of the recording medium. Although advantageous and generally effective for its intended purpose, if the same amount of transfer bias correction is applied to the DC component and the AC component, one of the DC component and the AC component is not corrected sufficiently, resulting in an image defect, such as unevenness of image density, or one of the DC component and the AC component is corrected excessively, causing electric discharge and thus white spots (partial absence of toner) in a resulting image
In view of the above, there is thus an unsolved need for an image forming apparatus capable of maintaining good transferability regardless of surface conditions of recording media.